Aquaporins are cell membrane proteins that act as molecular water channels to mediate the flow of water in and out of the cells. While there is some degree of passive diffusion or osmosis of water across cell membranes, the rapid and selective transport of water in and out of cells involves aquaporins. These water channels selectively conduct water molecules in and out of the cell, while blocking the passage of ions and other solutes, thereby preserving the membrane potential of the cell. Aquaporins are found in virtually all life forms, from bacteria to plants to animals. In humans, they are found in cells throughout the body.
Cerebral edema (CE) is a major contributor to stroke damage, as it can result in increased intracerebral pressure (ICP), a corresponding decrease in cerebral perfusion, and potentially permanent or fatal brain damage. Edema also contributes to CNS damage in, for example, traumatic brain and spinal cord injury, glioma, meningitis, acute mountain sickness, epileptic seizures, infections, metabolic disorders, hypoxia, water intoxication, hepatic failure, hepatic encephalopathy, diabetic ketoacidosis, abscess, eclampsia, Creutzfeldt-Jakob disease, and lupus cerebritis. Patients surviving the period of maximal ICP, usually the three days following a stroke or traumatic brain injury, are likely to survive. Unfortunately, only a few treatment options are available for CE, and these are of limited efficacy.
Hyponatremia, characterized by serum sodium levels ≤135 mM, is the most common form of electrolyte imbalance with hospitals nationwide reporting an incidence of 15-20%. The associated fluid retention is symptomatic of heart failure (HF), liver cirrhosis, nephrotic disorder, and syndrome of inappropriate antidiuretic hormone secretion (SIADH). Various diuretics are used to treat congestion associated with HF. By inhibiting the Na/K/Cl cotransporter in the thick ascending loop of Henle, loop diuretics cause natriuresis by decreasing Na+ and Cl− reabsorption from the urine. An alternative therapy for hyponatremia is the use of vasopressin receptor antagonists, which inhibit water reabsorption by inhibiting the vasopressin-induced trafficking of AQP2. Unfortunately, both loop diuretics and vasopressin receptor antagonists act indirectly toward a desired physiological outcome. An ideal drug would block water reabsorption directly, thus minimizing potential side-effects caused by upstream effectors, but no such drugs are currently known.
Epilepsy is a brain disorder characterized by recurrent seizures. Seizures occur because of disturbed brain activity resulting in some degree of temporary brain dysfunction. Seizures may cause uncontrollable shaking and loss of consciousness but, more commonly, a person experiencing a seizure stops moving or becomes unaware of what is happening. Anticonvulsants may be used to treat epilepsy, however anticonvulsants are not effective for all people with epilepsy.
AQP4 inhibitors may be of further utility for certain ailments where control of AQP4-medited water movement may augment neuroexcitation (by alteration of neuronal potassium homeostasis) and prove beneficial by reducing neuronal excitation, for example in ailments such as fibromyalgia, multiple sclerosis, migraines and seizures (in particular but not limited to seizures associated with epilepsy).
Ischemia is a condition characterized by an interruption or inadequate supply of blood to tissues. Retinal ischemia occurs due to a deficient supply of blood to the retina. Vascular occlusion, glaucoma, and diabetic retinopathy are associated with retinal ischemia and can produce retinal edema and ganglion cell death leading to visual impairment and blindness. AQP4 is expressed in the Muller cells in the retina. Due to relatively ineffectual treatment, retinal ischemia remains a common cause of visual impairment and blindness.
Myocardial ischemia is a condition caused by a blockage or constriction of one more of the coronary arteries, such as can occur with atherosclerotic plaque occlusion or rupture. Myocardial infarction, a heart attack, occurs when myocardial ischemia exceeds a critical threshold and overwhelms myocardial cellular repair mechanisms designed to maintain normal operating function and homeostasis. Myocardial infarction remains a leading cause of morbidity and mortality worldwide. Compounds effective in treating myocardial ischemia, myocardial ischemia/reperfusion injury, myocardial infarction, and congestive heart failure would be useful pharmaceuticals.
Phenylbenzamide compounds are known as pharmaceuticals. Phenylbenzamides include compounds such niclosamide (5-chloro-N-(2-chloro-4-nitrophenyl)-2-hydroxybenzamide), an antihemintic agent used to treat tapeworms, but are not known to have any effect on aquaporins. US Patent Publication US 2010/0274051 A1 (the contents of which are incorporated herein by reference) describe certain phenylbenzamides as being useful to inhibit NF-κB via selective inhibition of IKK-β, while U.S. Pat. No. 7,626,042 (also incorporated herein by reference) discloses O-acyl derivatives of such compounds, while U.S. Pat. No. 7,700,655 (also incorporated herein by reference) describe certain phenylbenzamides as being useful to treat allergic diseases. These patent applications, however, do not disclose anything about cerebral edema or water imbalance (aquaresis) or aquaporins.
In a 2004 paper, a group purportedly investigated the efficacy of N-(3,5-Bis-trifluoromethyl-phenyl)-5-chloro-2-hydroxy-benzamide in IκB phosphorylation blockade in a rat myocardial ischemia/reperfusion injury model. Onai, Y. et al., “Inhibition of IκB Phosphorylation in Cardiomyocytes Attenuates Myocardial Ischemia/Reperfusion Injury,” Cardiovascular Research, 2004, 63, 51-59. The group reported some activity. However, subsequently, the activity effect could not be confirmed and accordingly N-(3,5-Bis-trifluoromethyl-phenyl)-5-chloro-2-hydroxy-benzamide was not pursued for this indication in humans.
Prior to this invention, there are have been no known specific, validated inhibitors of aquaporins, for example AQP4 or AQP2. Certain antiepileptic or sulfonamide drugs (e.g., acetylsulfanilamide, acetazolamide, 6-ethoxy-benzothiazole-2-sulfonamide, topiramate, zonisamide, phenytoin, lamotrigine, and sumatriptan) were at one point reported to be possible inhibitors of AQP4, but this later proved to be incorrect. Yang, et al., Bioorganic & Medicinal Chemistry (2008) 16: 7489-7493. No direct inhibitors of AQP2 have been reported. The search for therapeutically useful aquaporin inhibitors has been hampered by a lack of effective high throughput screening assays, as well as by a lack of highly selective inhibitors to develop and validate the assays and to serve as positive controls or binding competitors.
There is a great need for improved approaches to treating and controlling diseases of water imbalance, such as edema, for example cerebral edema, and water retention and hyponatremia, as well as diseases such as epilepsy, retinal ischemia, myocardial ischemia, myocardial ischemia/reperfusion injury, myocardial infarction, myocardial hypoxia, congestive heart failure, sepsis, and neuromyelitis optica, as well as migraines.